
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;


/**
 * Default, non-distributed implementation of the Cache interface.
 * The algorithm for cache is as follows: a HashMap is maintained for fast
 * object lookup. Two linked lists are maintained: one keeps objects in the
 * order they are accessed from cache, the other keeps objects in the order
 * they were originally added to cache. When objects are added to cache, they
 * are first wrapped by a CacheObject which maintains the following pieces
 * of information:<ul>
 *
 * <li> The size of the object (in bytes).
 * <li> A pointer to the node in the linked list that maintains accessed
 * order for the object. Keeping a reference to the node lets us avoid
 * linear scans of the linked list.
 * <li> A pointer to the node in the linked list that maintains the age
 * of the object in cache. Keeping a reference to the node lets us avoid
 * linear scans of the linked list.</ul><p>
 *
 * To get an object from cache, a hash lookup is performed to get a reference
 * to the CacheObject that wraps the real object we are looking for.
 * The object is subsequently moved to the front of the accessed linked list
 * and any necessary cache cleanups are performed. Cache deletion and expiration
 * is performed as needed.
 * @param <K> <K>
 * @param <V> <V>
 * @author Matt Tucker
*/
public class DefaultCache<K, V> implements Map<K, V> {

    /**
     * The map the keys and values are stored in.
     */
    protected Map<K, CacheObject<V>> map;

    /**
     * Linked list to maintain order that cache objects are accessed
     * in, most used to least used.
     */
    protected LinkedList<K> lastAccessedList;

    /**
     * Linked list to maintain time that cache objects were initially added
     * to the cache, most recently added to oldest added.
     */
    protected LinkedList<K> ageList;

    /**
     * Maximum size in bytes that the cache can grow to.
     */
    private long maxCacheSize;

    /**
     * Maintains the current size of the cache in bytes.
     */
    private int cacheSize = 0;

    /**
     * Maximum length of time objects can exist in cache before expiring.
     */
    protected long maxLifetime;

    /**
     * Maintain the number of cache hits and misses. A cache hit occurs every
     * time the get method is called and the cache contains the requested
     * object. A cache miss represents the opposite occurence.<p>
     *
     * Keeping track of cache hits and misses lets one measure how efficient
     * the cache is; the higher the percentage of hits, the more efficient.
     */
    protected long cacheHits = 0L;
    /**
     * 记录缓存不中次数
     */
    protected long cacheMisses = 0L;

    /**
     * The name of the cache.
     */
    private String name;

    /**
     * Create a new default cache and specify the maximum size of for the cache in
     * bytes, and the maximum lifetime of objects.
     * @param maxLifetime the maximum amount of time objects can exist in
     *      cache before being deleted. -1 means objects never expire.
     */
    public DefaultCache(long maxLifetime) {
        this("", -1, maxLifetime);
    }
    
    /**
     * @param name a name for the cache.
     * @param maxSize the maximum size of the cache in bytes. -1 means the cache
     *      has no max size.
     * @param maxLifetime the maximum amount of time objects can exist in
     *      cache before being deleted. -1 means objects never expire.
 
     */
    public DefaultCache(String name, long maxSize, long maxLifetime) {
        this.name = name;
        this.maxCacheSize = maxSize;
        this.maxLifetime = maxLifetime;

        // Our primary data structure is a HashMap. The default capacity of 11
        // is too small in almost all cases, so we set it bigger.
        map = new HashMap<>(103);

        lastAccessedList = new LinkedList<>();
        ageList = new LinkedList<>();
    }

    /**
     * @see Map#put(Object, Object)
     * @param key key
     * @param value value
     * @return  V
     */
    public synchronized V put(K key, V value) {
        // Delete an old entry if it exists.
        V answer = remove(key);

        int objectSize = 1;
        try {
             objectSize = CacheSizes.sizeOfAnything(value);
        } catch (Exception e) {
             System.out.println(e.getMessage());
        }

        // If the object is bigger than the entire cache, simply don't add it.
        if (maxCacheSize > 0 && objectSize > maxCacheSize * .90) {
            System.out.println("Cache: " + name + " -- object with key " + key +
                    " is too large to fit in cache. Size is " + objectSize);
            return value;
        }
        cacheSize += objectSize;
        CacheObject<V> cacheObject = new CacheObject<>(value, objectSize);
        map.put(key, cacheObject);
        // Make an entry into the cache order list.
        // Store the cache order list entry so that we can get back to it
        // during later lookups.
        cacheObject.lastAccessedListNode = lastAccessedList.addFirst(key);
        // Add the object to the age list
        LinkedListNode<K> ageNode = ageList.addFirst(key);
        // We make an explicit call to currentTimeMillis() so that total accuracy
        // of lifetime calculations is better than one second.
        ageNode.timestamp = System.currentTimeMillis();
        cacheObject.ageListNode = ageNode;

        // If cache is too full, remove least used cache entries until it is
        // not too full.
        cullCache();

        return answer;
    }

    /**
     *  @see Map#get(Object)
     * @param key key
     * @return V
     */
    @SuppressWarnings("unchecked")
    public synchronized V get(Object key) {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        CacheObject<V> cacheObject = map.get(key);
        if (cacheObject == null) {
            // The object didn't exist in cache, so increment cache misses.
            cacheMisses++;
            return null;
        }

        // The object exists in cache, so increment cache hits. Also, increment
        // the object's read count.
        cacheHits++;
        //cacheObject.readCount++;

        // Remove the object from it's current place in the cache order list,
        // and re-insert it at the front of the list.
        cacheObject.lastAccessedListNode.remove();
        lastAccessedList.addFirst((LinkedListNode<K>) cacheObject.lastAccessedListNode);

        return cacheObject.object;
    }

    /**
     * @see Map#remove(Object)
     * @param key key
     * @return V
     */
    @SuppressWarnings("unchecked")
    public synchronized V remove(Object key) {
        if (map == null || key == null) {
            return null;
        }

        K k = (K)key;

        CacheObject<V> cacheObject = map.get(k);

        // If the object is not in cache, stop trying to remove it.
        if (cacheObject == null) {
            return null;
        }
        // remove from the hash map
        map.remove(key);
        // remove from the cache order list
        cacheObject.lastAccessedListNode.remove();
        cacheObject.ageListNode.remove();
        // remove references to linked list nodes
        cacheObject.ageListNode = null;
        cacheObject.lastAccessedListNode = null;
        // removed the object, so subtract its size from the total.
        cacheSize -= cacheObject.size;
        return cacheObject.object;
    }
    /** 
     *  @see Map#clear()
     */
    public synchronized void clear() {
        Object[] keys = map.keySet().toArray();
        for (int i = 0; i < keys.length; i++) {
            remove(keys[i]);
        }

        // Now, reset all containers.
        map.clear();
        lastAccessedList.clear();
        lastAccessedList = new LinkedList<>();
        ageList.clear();
        ageList = new LinkedList<>();

        cacheSize = 0;
        cacheHits = 0;
        cacheMisses = 0;
    }

    /**
     * @see Map#size()
     * @return  int
     */
    public int size() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return map.size();
    }

    /**
     *  @see Map#isEmpty()
     * @return  boolean
     */
    public boolean isEmpty() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return map.isEmpty();
    }

    /**
     * @see Map#values()
     * @return Collection<V> 
     */
    public Collection<V> values() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();
        return new CacheObjectCollection(map.values());
    }

    /**
     * 
     *Wraps a cached object collection to return a view of its inner objects
     */
    private final class CacheObjectCollection implements Collection<V> {
        /**
         * 缓存对象
         */
        private Collection<CacheObject<V>> cachedObjects;

        /**
         * 构造函数
         * @param cachedObjects  cachedObjects
         */
        private CacheObjectCollection(Collection<CacheObject<V>> cachedObjects) {
            this.cachedObjects = new ArrayList<>(cachedObjects);
        }

        /**
         * @see Collection#size()
         * @return int 
         */
        public int size() {
            return cachedObjects.size();
        }

        public boolean isEmpty() {
            return size() == 0;
        }

      
        /**
         * @see Collection#contains(Object)
         * @param o o
         * @return  boolean
         */
        public boolean contains(Object o) {
            Iterator<V> it = iterator();
            while (it.hasNext()) {
                if (it.next().equals(o)) {
                    return true;
                }
            }
            return false;
        }

        /**
         * @see Collection#iterator()
         * @return Iterator<V> 
         */
        public Iterator<V> iterator() {
            return new Iterator<V>() {
                private final Iterator<CacheObject<V>> it = cachedObjects.iterator();

                public boolean hasNext() {
                    return it.hasNext();
                }

                public V next() {
                    if (it.hasNext()) {
                        CacheObject<V> object = it.next();
                        if (object == null) {
                            return null;
                        } else {
                            return object.object;
                        }
                    } else {
                        throw new NoSuchElementException();
                    }
                }

                public void remove() {
                    throw new UnsupportedOperationException();
                }
            };
        }

        /**
         * @see Collection#toArray()
         * @return  Object[]
         */
        public Object[] toArray() {
            Object[] array = new Object[size()];
            Iterator<?> it = iterator();
            int i = 0;
            while (it.hasNext()) {
                array[i] = it.next();
            }
            return array;
        }


        /**
         * @see Collection#toArray(Object[])
         * @param a a
         * @param <T> v
         * @return  <T> T[]
         */
        @SuppressWarnings({ "unchecked" })
        public <T> T[] toArray(T[] a) {
            Iterator<T> it = (Iterator<T>) iterator();
            int i = 0;
            while (it.hasNext()) {
                a[i++] = it.next();
            }
            return a;
        }

        /**
         * @see Collection#containsAll(Collection)
         * @param c c
         * @return   boolean
         */
        public boolean containsAll(Collection<?> c) {
            Iterator<?> it = c.iterator();
            while (it.hasNext()) {
                if (!contains(it.next())) {
                    return false;
                }
            }
            return true;
        }

      
        /**
         * @param o 对象
         * @return  boolean
         */
        public boolean add(V o) {
            throw new UnsupportedOperationException();
        }

        /* (non-Javadoc)
         * @see java.util.Collection#remove(java.lang.Object)
         */
        /**
         *  @see Collection#remove(Object)
         * @param o o
         * @return  boolean
         */
        public boolean remove(Object o) {
            throw new UnsupportedOperationException();
        }

        
        /**
         * @see Collection#addAll(Collection)
         * @param coll coll
         * @return boolean
         */
        public boolean addAll(Collection<? extends V> coll) {
            throw new UnsupportedOperationException();
        }

        
        /**
         * @see Collection#removeAll(Collection)
         * @param coll coll
         * @return  boolean
         */
        public boolean removeAll(Collection<?> coll) {
            throw new UnsupportedOperationException();
        }

        /**
         * @see Collection#retainAll(Collection)
         * @param coll coll
         * @return  boolean
         */
        public boolean retainAll(Collection<?> coll) {
            throw new UnsupportedOperationException();
        }


        /** 
         *  @see Collection#clear()
         */
        public void clear() {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * @see Map#containsKey(Object)
     * @param key key
     * @return  boolean
     */
    public boolean containsKey(Object key) {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return map.containsKey(key);
    }


    /**
     * @see Map#putAll(Map)
     * @param map  map
     */
    public void putAll(Map<? extends K, ? extends V> map) {
        for (Iterator<? extends K> i = map.keySet().iterator(); i.hasNext();) {
            K key = i.next();
            V value = map.get(key);
            put(key, value);
        }
    }
    /**
     *  @see Map#containsValue(Object)
     * @param value value
     * @return  boolean
     */
    public boolean containsValue(Object value) {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        if (value == null) {
            return containsNullValue();
        }

        Iterator<?> it = values().iterator();
        while (it.hasNext()) {
            if (value.equals(it.next())) {
                 return true;
            }
        }
        return false;
    }

    /**
     * containsNullValue
     * @return  boolean
     */
    private boolean containsNullValue() {
        Iterator<?> it = values().iterator();
        while (it.hasNext()) {
            if (it.next() == null) {
                return true;
            }
        }
        return false;
    }
    /**
     * @see Map#entrySet()
     * @return Set
     */ 
    public Set<Entry<K, V>> entrySet() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        synchronized (this) {
            final Map<K, V> result = new HashMap<>();
            for (final Entry<K, CacheObject<V>> entry : map.entrySet()) {
                result.put(entry.getKey(), entry.getValue().object);
            }
            return result.entrySet();
        }
    }

    /* (non-Javadoc)
     * @see java.util.Map#keySet()
     */
    /**
     * @return Set<K>
     */
    public Set<K> keySet() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();
        synchronized (this) {
            return new HashSet<>(map.keySet());
        }
    }

    /**
     * Returns the name of this cache. The name is completely arbitrary
     * and used only for display to administrators.
     *
     * @return the name of this cache.
     */
    public String getName() {
        return name;
    }

    /**
     * Sets the name of this cache.
     *
     * @param name the name of this cache.
     */
    public void setName(String name) {
        this.name = name;
    }

    /**
     * Returns the number of cache hits. A cache hit occurs every
     * time the get method is called and the cache contains the requested
     * object.<p>
     *
     * Keeping track of cache hits and misses lets one measure how efficient
     * the cache is; the higher the percentage of hits, the more efficient.
     *
     * @return the number of cache hits.
     */
    public long getCacheHits() {
        return cacheHits;
    }

    /**
     * Returns the number of cache misses. A cache miss occurs every
     * time the get method is called and the cache does not contain the
     * requested object.<p>
     *
     * Keeping track of cache hits and misses lets one measure how efficient
     * the cache is; the higher the percentage of hits, the more efficient.
     *
     * @return the number of cache hits.
     */
    public long getCacheMisses() {
        return cacheMisses;
    }

    /**
     * Returns the size of the cache contents in bytes. This value is only a
     * rough approximation, so cache users should expect that actual VM
     * memory used by the cache could be significantly higher than the value
     * reported by this method.
     *
     * @return the size of the cache contents in bytes.
     */
    public int getCacheSize() {
        return cacheSize;
    }

    /**
     * Returns the maximum size of the cache (in bytes). If the cache grows larger
     * than the max size, the least frequently used items will be removed. If
     * the max cache size is set to -1, there is no size limit.
     *
     * @return the maximum size of the cache (-1 indicates unlimited max size).
     */
    public long getMaxCacheSize() {
        return maxCacheSize;
    }

    /**
     * Sets the maximum size of the cache. If the cache grows larger
     * than the max size, the least frequently used items will be removed. If
     * the max cache size is set to -1, there is no size limit.
     *
     * @param maxCacheSize the maximum size of this cache (-1 indicates unlimited max size).
     */
    public void setMaxCacheSize(int maxCacheSize) {
        this.maxCacheSize = maxCacheSize;
        // It's possible that the new max size is smaller than our current cache
        // size. If so, we need to delete infrequently used items.
        cullCache();
    }

    /**
     * Returns the maximum number of milleseconds that any object can live
     * in cache. Once the specified number of milleseconds passes, the object
     * will be automatically expried from cache. If the max lifetime is set
     * to -1, then objects never expire.
     *
     * @return the maximum number of milleseconds before objects are expired.
     */
    public long getMaxLifetime() {
        return maxLifetime;
    }

    /**
     * Sets the maximum number of milleseconds that any object can live
     * in cache. Once the specified number of milleseconds passes, the object
     * will be automatically expried from cache. If the max lifetime is set
     * to -1, then objects never expire.
     *
     * @param maxLifetime the maximum number of milleseconds before objects are expired.
     */
    public void setMaxLifetime(long maxLifetime) {
        this.maxLifetime = maxLifetime;
    }

    /**
     * Clears all entries out of cache where the entries are older than the
     * maximum defined age.
     */
    protected void deleteExpiredEntries() {
        // Check if expiration is turned on.
        if (maxLifetime <= 0) {
            return;
        }

        // Remove all old entries. To do this, we remove objects from the end
        // of the linked list until they are no longer too old. We get to avoid
        // any hash lookups or looking at any more objects than is strictly
        // neccessary.
        LinkedListNode<K> node = ageList.getLast();
        // If there are no entries in the age list, return.
        if (node == null) {
            return;
        }

        // Determine the expireTime, which is the moment in time that elements
        // should expire from cache. Then, we can do an easy to check to see
        // if the expire time is greater than the expire time.
        long expireTime = System.currentTimeMillis() - maxLifetime;

        while (expireTime > node.timestamp) {
            // Remove the object
            remove(node.object);

            // Get the next node.
            node = ageList.getLast();
            // If there are no more entries in the age list, return.
            if (node == null) {
                return;
            }
        }
    }

    /**
     * Removes objects from cache if the cache is too full. "Too full" is
     * defined as within 3% of the maximum cache size. Whenever the cache is
     * is too big, the least frequently used elements are deleted until the
     * cache is at least 10% empty.
     */
    protected final void cullCache() {
        // Check if a max cache size is defined.
        if (maxCacheSize < 0) {
            return;
        }

        // See if the cache size is within 3% of being too big. If so, clean out
        // cache until it's 10% free.
        int desiredSize = (int)(maxCacheSize * .97);
        if (cacheSize >= desiredSize) {
            // First, delete any old entries to see how much memory that frees.
            deleteExpiredEntries();
            desiredSize = (int)(maxCacheSize * .90);
            if (cacheSize > desiredSize) {
                long t = System.currentTimeMillis();
                do {
                    // Get the key and invoke the remove method on it.
                    remove(lastAccessedList.getLast().object);
                } while (cacheSize > desiredSize);
                t = System.currentTimeMillis() - t;
                System.out.println("Cache " + name + " was full, shrinked to 90% in " + t + "ms.");
            }
        }
    }

    /**
     * Wrapper for all objects put into cache. It's primary purpose is to maintain
     * references to the linked lists that maintain the creation time of the object
     * and the ordering of the most used objects.
     */
    private static class CacheObject<V> {

        /**
         * Underlying object wrapped by the CacheObject.
         */
        public V object;

        /**
         * The size of the Cacheable object. The size of the Cacheable
         * object is only computed once when it is added to the cache. This makes
         * the assumption that once objects are added to cache, they are mostly
         * read-only and that their size does not change significantly over time.
         */
        public int size;

        /**
         * A reference to the node in the cache order list. We keep the reference
         * here to avoid linear scans of the list. Every time the object is
         * accessed, the node is removed from its current spot in the list and
         * moved to the front.
         */
        public LinkedListNode<?> lastAccessedListNode;

        /**
         * A reference to the node in the age order list. We keep the reference
         * here to avoid linear scans of the list. The reference is used if the
         * object has to be deleted from the list.
         */
        public LinkedListNode<?> ageListNode;

        /**
         * Creates a new cache object wrapper. The size of the Cacheable object
         * must be passed in in order to prevent another possibly expensive
         * lookup by querying the object itself for its size.<p>
         *
         * @param object the underlying Object to wrap.
         * @param size   the size of the Cachable object in bytes.
         */
        public CacheObject(V object, int size) {
            this.object = object;
            this.size = size;
        }
    }
}
